Android backbutton

martin_ky

Hi there. The back button functionality works nicely in the Qt 5.1 developer preview without need for any tricks and behaves "according to specification":http://developer.android.com/reference/android/app/Activity.html (scroll to onKeyDown method).

According to Android documentation, the back button event is delivered via Activity.onKeyUp() which is translated as QEvent::KeyRelease with key code Qt::Key_Back.

To prevent your app from quitting, you have to handle (accept) this event. For example, in QML you would do something like this:

@
Rectangle {
focus: true // important - otherwise we'll get no key events

Keys.onReleased: {
    if (event.key == Qt.Key_Back) {
        console.log("Back button captured - wunderbar !")
        event.accepted = true
    }
}

}
@

If you want to handle it on C++ level, override keyReleaseEvent() of your main Window and do the same in there.

That being said, how you implement the "come back at previous stack page" is entirely up to you.

LangstoniusRex

Thanks for explaining :) It is a bit odd that it somehow works its way to quit.

erik.ridderby

I managed to catch the android back button in QML:

@ApplicationWindow {
onClosing: {
close.accepted = false
if (contextMenuManager.menuVisible)
contextMenuManager.menuVisible = false
}
}@

Rizzer

Keys.onReleased catches the Android back button in some instances.

It doesn't seem to work with ListView though. focus must be directed somewhere where I haven't anticipated.

Here is a sample main.qml with a ListView, which does not catch the Android back button:
@
import QtQuick 2.3
import QtQuick.Controls 1.2
import QtQuick.Layouts 1.1

ApplicationWindow {
id: appWindow
visible: true
width: 640
height: 480
title: qsTr("Hello World")

ListModel {
    id: longModel

    Component.onCompleted: {
        for ( var i=1; i<=100; i++ )
            append({"testName": i})
    }
}


ListView {
    anchors.fill: parent
    id: listView
    model: longModel

    delegate:
        Rectangle {
            height: 80
        width: parent.width

        Text {
            id: textComponent
            text: testName
        }
        Keys.onReleased: {
            if (event.matches(StandardKey.Back)) {
                console.log("back caught by delegate");
                event.accepted = true;
            }
        }
    }

    focus: true
    Keys.onReleased: {
        if (event.matches(StandardKey.Back)) {
            console.log("back caught by listview");
            event.accepted = true;
        }
    }

}

}
@

Rizzer

Keys.onReleased catches the Android back button in some instances.

It doesn't seem to work with ListView though. focus must be directed somewhere where I haven't anticipated.

Here is a sample main.qml with a ListView, which does not catch the Android back button:
@
import QtQuick 2.3
import QtQuick.Controls 1.2
import QtQuick.Layouts 1.1

ApplicationWindow {
id: appWindow
visible: true
width: 640
height: 480
title: qsTr("Hello World")

ListModel {
    id: longModel

    Component.onCompleted: {
        for ( var i=1; i<=100; i++ )
            append({"testName": i})
    }
}


ListView {
    anchors.fill: parent
    id: listView
    model: longModel

    delegate:
        Rectangle {
            height: 80
        width: parent.width

        Text {
            id: textComponent
            text: testName
        }
        Keys.onReleased: {
            if (event.matches(StandardKey.Back)) {
                console.log("back caught by delegate");
                event.accepted = true;
            }
        }
    }

    focus: true
    Keys.onReleased: {
        if (event.matches(StandardKey.Back)) {
            console.log("back caught by listview");
            event.accepted = true;
        }
    }

}

}
@

Mr_Ada

Can someone post a C++ version of this? I have tried to convert it and I am still not seeing the back key.

Never mind.... Here is a C++ version I was able to get to work....

@
#include <QDebug>

// regular MainWindow stuff...

void MainWindow::keyPressEvent (QKeyEvent* event) {

if (event->key () == Qt::Key_Back)
qDebug () << "<<keyPressEvent>> " <<
"[[Back button]]";
else if (event->key () == Qt::Key_HomePage)
qDebug () << "<<keyPressEvent>> " <<
"[[Home button]]";
else if (event->key () == Qt::Key_Menu)
qDebug () << "<<keyPressEvent>> " <<
"[[Menu button]]";
else if (event->key () == Qt::Key_unknown)
qDebug () << "<<keyPressEvent>> " <<
"[[Unknown button]]";
event->accept ();
}
@

Mr_Ada

Can someone post a C++ version of this? I have tried to convert it and I am still not seeing the back key.

Never mind.... Here is a C++ version I was able to get to work....

@
#include <QDebug>

// regular MainWindow stuff...

void MainWindow::keyPressEvent (QKeyEvent* event) {

if (event->key () == Qt::Key_Back)
qDebug () << "<<keyPressEvent>> " <<
"[[Back button]]";
else if (event->key () == Qt::Key_HomePage)
qDebug () << "<<keyPressEvent>> " <<
"[[Home button]]";
else if (event->key () == Qt::Key_Menu)
qDebug () << "<<keyPressEvent>> " <<
"[[Menu button]]";
else if (event->key () == Qt::Key_unknown)
qDebug () << "<<keyPressEvent>> " <<
"[[Unknown button]]";
event->accept ();
}
@

OE0214

on Android, I did this:

void MainWindow::keyPressEvent(QKeyEvent *event)
{
	if (event->key() == Qt::Key_Back)
		qDebug() << "[[Back button]]";
	else if (event->key() == Qt::Key_Menu)
		qDebug() << "[[Menu button]]";
	else if (event->key() == Qt::Key_TopMenu)
		qDebug() << "[[Top menu button]]";
	else
		qDebug() << "[[some other button]]";

	event->accept();
}

I get a debug message for the back button, but not for the menu button...I also tried Qt::Key_TopMenu, but still nothing

nor do I get the "some other button" debug message when the menu button is pressed, so this method is not even being called when I press the menu button

this message must be handled somewhere else

can anyone help?

Thanks

uCampaign

In Qt 5.4.2 this is what i use and it works fine:

ApplicationWindow {
....
onClosing: {
if (Qt.platform.os == "android") {
close.accepted = false;
if (stack.depth > 1) stack.pop();
}
}
}

bbaker

C++ code. Gives the desired/standard effect on Android. Use a QWidget with a QStackedLayout in the main window.

    stackedLayout = new QStackedLayout;
    QWidget *c = new QWidget();
    first = new FirstForm(this );

    stackedLayout->addWidget( first );

    c->setLayout( stackedLayout );
    setCentralWidget( c );

Override the closeEvent in the header:

protected:
    void closeEvent(QCloseEvent *event);

Implement the code in the body:

void MainWindow::closeEvent(QCloseEvent *event) {

if( stackedLayout->currentIndex() > 0 ) {
    QWidget *top = stackedLayout->currentWidget();
    stackedLayout->removeWidget( top );

    event->ignore();
}

}

// QED

seyed

@uCampaign this is my enhanced version:

Window {
    //...
    onClosing: {
        if (Qt.platform.os === "android" || Qt.platform.os === "ios") {
            close.accepted = false;
            if (menu.isShown) menu.hide();
            else if (stackView.depth > 1) stackView.pop();
            else Qt.quit();
        }
    }
}

Thank you :)

kgregory

Does anyone know of a full example of an app that captures the back button? Maybe a github repo? None of these code snippets are working for me and I'm not sure why.